The voltage controlled variable resistor is a very useful component in the design of many applications, for example, the voltage controlled gain cell, AGC circuit, automatic gain control, and the like.
Reference is now made to FIG. 1 which shows a diagram of a prior art voltage controlled variable resistor circuit 10. The circuit 10 is implemented using a forward biased diode 12. The circuit 10 may typically be used in an RF application as an automatic gain control (AGC) circuit. The diode 12 is typically a PIN diode configured for use as a variable resistor. The equivalent resistance is controlled by the current through the diode 12. Equation 1 presents the diode voltage:
      V    D    =                    V        T            ·      ln        ⁢                  I        D                    I        S            Equation 2 presents the diode resistance:
                              r          D                =                ⁢                              ∂                          V              D                                            ∂                          I              D                                                              =                ⁢                              V            T                    ·                                    I              S                                      I              D                                ·                      1                          I              S                                                              =                ⁢                              V            T                                I            D                              
A problem with the circuit 10 of FIG. 1 is that the circuit 10 can be only used in small signal condition (in the range of mV or μV). With a large signal voltage, however, the equation of rD is no longer valid and a large distortion in the signal is introduced.
Reference is now made to FIG. 2 which shows a diagram of a prior art voltage controlled variable resistor circuit 20. The circuit 20 is implemented using a MOS transistor 22 operating in the triode region. In the triode region, the I/V characteristic of the MOS transistor 22 looks like a voltage (VGS) controlled variable resistor. The equivalent resistance is given by Equation 3:
      R          E      ⁢                          ⁢      Q        =      1                  K        ·                  W          L                    ⁢              (                              V                          G              ⁢                                                          ⁢              S                                -                      V                          T              ⁢                                                          ⁢              H                                      )            In order to guarantee linearity with respect to the equivalent resistance, the voltage between the source and drain of the MOS transistor 22 must be kept very small (for example, tens of mV). This circuit 20 of FIG. 2 accordingly suffers from the same problem as circuit 10 of FIG. 1.
Reference is now made to FIG. 3 which shows a diagram of a prior art voltage controlled variable resistor circuit 30. The circuit 30 is implemented using a plurality of series connected resistors 32. A plurality of switches (for example, MOS devices) 34 are coupled to selectively shunt around one or more of the resistors 32 so as to change the end-to-end resistance value of the circuit 30. The switches 34 are digitally controlled. This type of variable resistor circuit 30 is suitable for use with large signal levels. However, because of the digital control, the resistance variation is discontinuous.
There is a need in the art for a voltage controlled variable resistor suitable for continuous and linear resistance variation and use with large scale signals.